The polymerization of tetrafluoroethylene, hereinafter called "TFE", in aqueous emulsion is a known technique which makes it possible to obtain dispersions of polytetrafluoroethylene (PTFE) in water which have found numerous industrial applications; in particular, the coating of metals or other substrates, the impregnation of fibers and fabrics, the preparation of films and varnishes, and the production by coagulation of powders for lubricated extrusion.
In the processes which are presently known, the PTFE dispersions are prepared at constant temperature by placing in contact with each other, in a pressure-resistant autoclave, the gaseous monomer, or a mixture of monomers in the case of the preparation of copolymers, and an aqueous solution kept at the constant polymerization temperature and containing an initiator capable of forming free radicals at the polymerization temperature, possibly a buffer in order to keep the medium at a given pH, and an emulsifier which generally is an alkaline or ammonium salt of a carboxylic or sulfonic acid whose carbon chain carries only fluorine atoms and possibly chlorine atoms. Under these conditions, the dispersions obtained are not very stable and it is hardly possible to exceed a PTFE concentration of 25 to 30% by weight.
The fact of not being able to exceed degrees of conversion corresponding to 25 to 30% by weight of final PTFE concentration results in obtaining products whose molecular weights are too low. In fact, it is above all at the end of polymerization, that is to say at high conversion degrees, that the molecules of highest molecular weight are being formed. Consequently, everything else being equal, the higher the final concentrations which are achieved, the higher the mean molecular weight of the powder and the better its properties on application.
Among the different possibilities for increasing the molecular weight on polymerization, there are those possibilities which permit this improvement without an increase in the degree of conversion and then there are those possibilities which, while permitting an increase in the degree of conversion, at the same time bring about an increase in the molecular weight. These latter possibilities are the most interesting ones, since they improve the productivity of the material.
An increase in the molecular weight can, for instance, be achieved by diminishing the quantity of initiators used, but this presents the drawback of slowing the polymerization and, consequently, of increasing the time during which the dispersion is subjected to agitation and, thus, of obtaining less stable dispersions or still lower PTFE concentrations. The reaction rate can, for instance, also be increased, either by increasing the monomer pressure, but technological problems and problems of material costs are encountered very soon; or by increasing agitation, but there again is the risk of further destabilizing the dispersion; or by increasing the quantity of initiators, which has the effect of annulling the gain contributed by the increase in the reaction rate.
An increase in the molecular weights can be achieved, without changing the quantities of initiators, by an improvement of the conversion into PTFE, that is to say by increasing the final concentration of the dispersions.
A considerable improvement in the results was obtained according to French Pat. No. 1,019,304 by adding to the polymerization medium, as a stabilizer, from 0.1 to 12% by weight of the aqueous dispersion of a saturated hydrocarbon containing more than 12 carbon atoms and being liquid under the conditions of the polymerization. This process presents several drawbacks. It makes cleaning of the polymerization autoclave difficult, and this results in a loss of time which is not very compatible with industrial production, especially when a paraffin hydrocarbon which is solid at ambient temperature is used. Moreover, if the hydrocarbon used is liquid, its total elimination from the dispersion requires rather lengthy settling and decantations which in practice lead to the loss of a significant quantity of PTFE. Finally, if the elimination of the hydrocarbon is not perfect, residual traces of this product contaminate the finished product. In addition, the saturated hydrocarbons commonly employed are often more or less pure mixtures which can change with time and thus cause a polymerization irregularity and, above all, mixtures which possess a slight transfer of activity which increases with increasing temperature and prevents products of high molecular weight from being obtained.
Another improvement proposed by Belgian Pat. No. 678,637 consists in using a halogenated paraffin hydrocarbon having from 1 to 3 carbon atoms; in particular, 1,1,2-trifluoro-1,2,2 trichloroethane, as the stabilizer. However, the use of this process is limited by the obligation of working at polymerization temperatures between 5.degree. C. and 30.degree. C. and by the necessity of using large quantities of chlorofluorinated hydrocarbon, from 50 to 500 g per liter of aqueous dispersion, and this requires a recovery of the stabilizer by decantation of the dispersion and in practice entails a loss of stabilizer.
Another improvement proposed by French Pat. No. 2,123,703 consists in using as the stabilizer extremely low quantities of cyclic or acyclic halogenated saturated hydrocarbons having less than 20 carbon atoms and being non-copolymerizable with TFE, or mixtures of these products. Concentrated dispersions of PTFE exhibiting a very great purity and an excellent stability are obtained, but it is not possible to obtain products of high molecular weight, since the additives have a high chain transfer activity.
British Pat. No. 1,545,675 proposes a polymerization carried out in part at a temperature being kept constant and in part at another temperature. The goal sought after then is an increase of the amorphism index for the polymer formed in the second part of the polymerization. According to the graph attached to this patent, if the first part of the polymerization is carried out at 70.degree. C. or more, the second part must be done at a higher temperature; but if the first part is done at a lower temperature, then the second part must be done at an even lower temperature. Furthermore, this polymerization uses paraffin wax as the stabilizer and the final concentration does not exceed 25% by weight.